The present invention relates generally to playback circuits for capacitance disc records.
It is known in the art that the reproduction of capacitance disc records involves the use of a stylus formed of a diamond with an electrode attached thereto to form a variable capacitance between it and the conductive layer of a capacitance disc record on which information is recorded in the form of a series of microscopic pits. This geometric variation is detected as a variation in capacitance. In current practice, the capacitance between the stylus electrode and the record's conductive layer forms part of a coaxial resonator to which microwave energy is coupled by a coupling circuit from a 1-GHz oscillator. The coaxial resonator acts as an amplitude modulator, so that the microwave energy is modulated in amplitude with the capacitance variation on the disc record. The output of the coaxial resonator is applied to an amplitude demodulator to detect a signal representing the capacitance variation.
A typical example of prior art capacitance detectors is shown specifically in FIG. 1, as comprising a metal casing 7 which is grounded and serves as an outer conductor, the casing being separated by a conductive partition 8 to define a first chamber for generation of the high frequency energy and a second chamber for amplitude modulation and demodulation. The oscillator 1 comprises a strip line conductor 10 which is formed in the first chamber on the surface of a dielectric support 9 and extends along one side of the metal casing 7. The conductor 10, which acts as an inner conductor of the oscillator, is capacitively coupled by a trimmer capacitor 11 to one end wall of the casing, the other end of the conductor 10 being capacitively coupled by a coupling capacitor 12 to a transistor 13 disposed near the opposite end wall of the casing 7, the transistor 13 being connected to a known bias supply circuit, not shown, mounted on the dielectric support 9. The trimmer capacitor is used to adjust the generated frequency to a predetermined value (1 GHz, for example). The microwave energy generated in the oscillator 1 is coupled by a coupling circuit 2 formed by a loop 20 to the second chamber. This coupling loop has a first elongated section extending from the oscillator-side of the partition 8 in parallel with the conductor 10 to establish an inductive coupling therewith and a second elongated section terminating at the other side of the partition, these elongated sections being coupled by an intermediate section extending through an opening provided in the partition 8. A coaxial resonator 3 and an amplitude demodulator 4 are located in the second chamber. The resonator 3 is formed by a strip line conductor 30 which is in parallel with the second elongated section of the loop so that the microwave energy is coupled from the oscillator 1 via the coupling circuit 2 to the resonator 3. One end of the conductor 30 is coupled through an opening in the casing 7 by a connecting wire 32 to the electrode of a capacitance detection stylus to supply it with microwave energy and the other end of which is coupled capacitively by means of a trimmer capacitor 31 to the casing 7. The microwave energy inductively coupled by loop 20 to the conductor 30 is amplitude modulated with the capacitance variation on the disc record. The amplitude-modulated energy is coupled by a coupling line conductor 5 to the amplitude demodulator 4 which is in the configuration of a peak detector formed by a diode 40 having its anode coupled to one end of the conductor 5 and its cathode coupled by a resistor 41 to the casing, the junction between diode 40 and resistor 41 being coupled to a feedthrough capacitor 42 having an output terminal 6.
The magnitude of the detected capacitance signal is proportional to the magnitude of the input microwave energy provided that the input frequency is kept precisely within a predetermined frequency range. To improve the signal-to-noise ratio at the output terminal 6, the amount of coupling between the oscillator 1 and resonator 3 could be increased to raise the input energy to the stylus. However, this results in a reduction in the carrier-to-noise ratio of the oscillator 1 due to the finite loaded Q-value of the oscillator and therefore no improvement is made in the signal-to-noise ratio. Additionally, the optimum value of the coupling is dependent on the manner of manufacture of the stylus electrode. For example, two types or styli are known in the art, one having an electrode formed by the deposition of hafnium or titanium and the other having an electrode formed by the transformation of the constituent carbon of the diamond into a conductive layer using an intense laser beam. Due to different tendencies to wear by contact with the record surface, these styli may differ in spacing between the lowermost end of the electrode and the stylus' contact face with the record surface and in the electrode's resistance. To overcome such shortcomings the prior art capacitance detector would involve time consuming work with which the spacing between the coupling loop 20 and its associated conductors is adjusted.
In addition, the conductor 5, the stray capacitances of the diode 40 and resistor 41, and the capacitor 42 constitute a resonant circuit having a resonant frequency higher than that of the resonator 3. This resonant circuit and the resonator 3 form a double-tuned circuit. Therefore, mutual interaction occurs between the resonator 3 and demodulator 4. However, due to manufacturing tolerances resulting in detector-to-detector variations in the capacitance values of diode 40 and capacitor 42, the resonant frequency of the amplitude demodulator 4 would vary in a wide range between different detectors, resulting in a wide range of detector output variations.